SHARM EL SHEIKH, Egypt—The Egyptian-led probe into what caused the crash of a Russian passenger plane is bogged down in secrecy and squabbling, causing concern among safety experts that key evidence is being compromised.

Nearly two weeks after the Airbus A321 broke apart in midair on Oct. 31, most of the debris remains scattered over miles of desert in the Sinai Peninsula even though it could contain critical clues.